Optical absorption and ESR spectra of mixed conductive glasses in Li2O-B2O3-WO3 system

Optical absorption and ESR spectra of mixed conductive glasses in Li2O-B2O3-WO3 system

SOLID STATE Solid State Ionics 57 (1992) 169-172 North-Holland IOIIICS Optical absorption and ESR spectra of mixed conductive glasses in L i 2 0 - B...

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SOLID STATE

Solid State Ionics 57 (1992) 169-172 North-Holland

IOIIICS Optical absorption and ESR spectra of mixed conductive glasses in L i 2 0 - B 2 0 3 - W O 3 system Peng-nian Huang i Xi-huai Huang Shanghai Institute of Ceramics, Academia Sinica, I295 Dingxi Road, Shanghai 200050, P.R. China

and Fu-xi Gan Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, P.O. Box 8216, Shanghai, P.R. China

Received 21 October 1991; accepted for publication 30 November 1991

In this work, the optical absorption and ESR spectra of mixed conductive lithium borotungstate glasses are measured. The spin density as well as the weight percent of W 6+ ions are calculated. The electronic structure of tungsten ions and the network structure of the glasses are analyzed. It is found that both tungsten ions W 5+ and W 6+ co-exist in the glasses with Li20 content less than 35 mol% and there are predominately W 6+ ion in the glasses with Li20 content > 35 mol%. The optical absorption caused by W 5÷ ions decreases with increasing LizO content and rapidly increases with increasing WO3 content. The spin density, however, anomalously weakens with increasing WO3 content so as to be zero finally in contrast with the optical properties of the glasses. It follows that the increase of WO3 content must lead to the polymerization of tungsten-oxygen polyhedra and therefore, there must exist two electrons with paired spins in the 5d-state of tungsten ions in the larger tungstate clusters.

I. Introduction

In recent years great a t t e n t i o n has b e e n p u t o n fast ion c o n d u c t i v e a n d m i x e d c o n d u c t i v e glasses [ 1,2 ]. T h e f o r m e r are p r o m i s i n g to be u s e d in solid state b a t t e r i e s a n d sensors, w h e r e a s the latter are e x p e c t e d to b e c o m e c a t h o d e a n d e l e c t r o c h r o m i c materials. A c c o r d i n g to o u r p r e v i o u s s t u d i e s . [ 3 ] , the glassf o r m i n g region in L i 2 0 - B 2 0 3 - W O 3 system consists o f b o t h acidic a r e a w i t h L i 2 0 c o n t e n t less t h a n 35 tool% in w h i c h the glasses are m i x e d c o n d u c t i v e a n d the basic area with L i 2 0 c o n t e n t higher t h a n 35 m o l % in w h i c h the glasses are essentially i o n c o n d u c t i v e . In o r d e r to f u r t h e r the studies o n the r e l a t i o n s o f the electrical c o n d u c t i o n w i t h t h e p r o p e r t i e s o f t u n g s t e n Paper presented at SSI-8, Lake Louise, Canada, October 2026, 1991. To whom all correspondence is to be sent to Chemistry Department, St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 1C0

ions, in this w o r k the optical a b s o r p t i o n a n d the E S R spectra o f the l i t h i u m b o r o t u n g s t a t e glasses are studied, the e l e c t r o n i c structure o f t u n g s t e n ions as well as the n e t w o r k s t r u c t u r e o f the glasses are f u r t h e r analyzed.

2. Experimental and results T h e p r e p a r a t i o n m e t h o d s o f glasses w e r e the s a m e as d e s c r i b e d earlier in [ 3 ] . T h e s a m p l e s for o p t i c a l m e a s u r e m e n t s were well p o l i s h e d a n d w e r e 1 m m thick. T h e o p t i c a l m e a s u r e m e n t s w e r e p e r f o r m e d o n a B e c k m a n n 5270 S p e c t r o p h o t o m e t e r . T h e effects o f L i 2 0 a n d WO3 c o n t e n t s on the optical a b s o r p t i o n o f glasses are s h o w n in figs. 1 a n d 2, respectively. S i m i l a r to the i n d i r e c t t r a n s i t i o n in crystalline s e m i c o n d u c t o r s , the o p t i c a l a b s o r p t i o n c o e f f i c i e n t s o f glasses n e a r the abs o r p t i o n edge fit the f o l l o w i n g r e l a t i o n (fig. 3):

0167-2738/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

170

P. Huang et al. / ESR spectra o f mixed conductive glasses

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300

400

500

600

700

800

WAVELENGTH(nm)

Fig. 1. The absorption spectra of x L i 2 0 ( 9 5 - x ) B 2 0 3 5 W O 3 glasses• ( 1 ) x = 15, (2) x=20, (3) x=25, (4) x=30, (5) x=35, (6) x=40, (7) x=45.

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Fig. 2. The absorption spectra of 25Li20. (75-x)B203.xWO3 glasses.

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Fig. 3. The plot of (o~hu) ~/2 versus hu for 4 0 L i 2 O ' 5 5 B 2 0 3 " 5 W O 3 glass.

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WAVELENGTH(nm)

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CuSO4.5H20 was used as standard reference for quantitative measurements. In the ESR spectra of glasses in the acidic area, a broad ESR absorption line was observed, as shown in fig. 4. No superfine structure was obtained in the ESR spectra of all the glasses studied at whether room temperature or liquid nitrogen temperature. The spin densities of some glasses are listed in table 2.

x

o

3

2,

here a is the optical absorption coefficient, Eg is the optical bandgap, h is Planck's constant and u is photon frequency. The optical bandgap Eg determined from plots of (c~h u)1/2 versus h u along with the colours and the electronic transference numbers of corresponding glasses are listed in table 1. The electronic transference numbers o f the glasses were measured with Wagner's polarization method [4]. The glass samples for ESR measurements were cylinders with the same size and were placed in a quartz glass tube. The ESR spectra were investigated in the X-band on a J E S - F E I X spectrometer with computerized analysis system under both room temperature and liquid nitrogen temperature.

3. Discussions

3.1. The valences of tungsten ions and their distribution As a transition element, tungsten ions may exist in glasses in different valences, mainly in W s+ and W 6+ states, depending on host glass and preparation conditions. The ESR spectra of the W 5+ ions can be roughly described by the spin Hamilton of 5d' ion in a ligand field of axial symmetry [5],

H=g,flIt~S= +g±fl(H~S~ + H,,Sv) , where fl is Bohr magnetron. In this work, an ESR signal with g-factor 1.70 was observed on the spectra of glasses with Li20 content less than 35 tool% at room temperature. This signal has been previously obtained in a-WO3 film [5] and tungstate phosphate glasses [6] and has been assigned to W 5+ ions. At low temperature, the signal was stronger and very

P. Huang et al. /ESR spectra of mixed conductive glasses

171

Table 1 The optical bandgap (Es) electronic transference number and colour of some glasses. Glass composition (mol)

Li20

B203

WO3

15 15 20 20 25 25 25 25 30 30 30 30 30 35 40 45

82 80 75 70 70 65 60 55 65 60 55 50 45 60 55 50

3 5 5 10 5 10 15 20 5 10 15 20 25 5 5 5

= .

H

Fig. 4. The ESR signalof W5÷ ions in 25Li20.70B203'5WO3glass.

Table 2 The apparent spin density and percent of W5+ ions of some glasses. Glass composition (mol)

Li20

B203

WO 3

20 25 30 25 25

75 70 65 65 60

5 5 5 10 15

Spin density (spins/g)

W5+ (%)

3.71 >( 1017 2.86× 1017 2.01X 1017 2.33 X 1017

0.0135 0.0104 0.0073 0.0085 0.0035

9.57× 1016

Es(eV)

te

Color

3.42 2.61 3.44 3.36 4.18 3.62 3.36 2.04 3.94 3.75 3.66 3.26 2.20

0.072 0.862 0.222 0.332 0.100 0.126 0.235 0.345 0.097 0.222 0.288 0.310 0.322 0.059 0.006 0.000

dark blue dark blue blue-black blue-black brown grey-blue dark-blue blue-black yellow brown brown red-brown deep brown colourless colourless colourless

slightly shifted to lower value. The strength of the ESR signal and the spin density (table 2) markedly decrease with increasing Li20 content so that there is no ESR absorption observed in the glasses with Li20 content higher than 35 mol%. It follows that the density of W 5+ ions decreases with increasing Li20 content, the W 5+ a n d W e+ ions must co-exist in the glasses with Li20 content lower than 35 mol% and W 6+ ions, the highest oxidization state, probably predominately exist in the glasses containing higher Li20 content. This result is consistent with the optical and electrical properties of the glasses. In fact, the glasses with Li20 content lower than 35 mol% are differently coloured and most of them are mixed conductors (table 1 ), the glasses with higher Li20 content are colourless and ion conductors [ 3]. 3.2. The optical absorption o f W 5+ ions Fig. 1 shows the effect of Li20 content on the optical properties of glasses. The absorption of W 5+ ions in the near IR and visible regions decreases and the absorption edge shifts towards higher energy with increasing Li20 content. For glasses in the basic area, the absorption in near IR and visible regions disappeared, the absorption edge is in high energy and almost does not change with Li20 content. The op-

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P. Huang et aL / ESR spectra of mixed conductive glasses

tical absorption of W 5+ ions in the glasses is rather broad and complex, as observed by Koffyberg et al. [6] in zinc borotungstate glasses. Fig. 5 shows the variation of absorption coefficient of 20Li:O. 75B203 glass with photon energy. Besides the absorption at 1.91 eV, in fact, there is another absorption at about 3.1 eV. The former is related to the b*-e* transition of W 5+ ions and in this case the W 5+ ions are "isolated" ones in the deformed octahedral oxygen ligand with C4v symmetry, as observed in a-WO3 film [ 7 ]. The latter, according to Kopp et al. [ 8 ], results from the transition between 5d localized state and conduction band.

3.3. Anomaly of the variation of spin density with WO~ content As shown in fig. 2, the optical absorption caused by W 5+ ions rapidly increases with increasing WO3 content, accompanied by large enhancement of electronic conduction [3]. Both indicate that the density of W 5+ ions markedly increases with increasing WO3 content. It is assumed that the integrated intensity of the ESR signal is proportional to the concentration of W 5+ ions. The intensity of the ESR signal as well as the spin density (table 2 ) of the glasses, however, sharply decreases with increasing WO3 content so as to approach zero finally, in contrast with the results from optical and electrical properties. It follows from this anomalous phenomenon that at low WO3 content, "isolated" tungsten ions exist in the interstitial position of glass network, when the WO3 content increases polymerization of tungsten-oxygen polyhedra e.g. W5+O6, W 6 + O 6 and WO4 must occur,

the same as the polymerization of WO6 octahedra in a-WO3 film [5] and therefore, there must exist two electrons with paired spins in the 5d-state of tungsten ions in the polymerized larger tungstate clusters.

4. Conclusions The glass-forming region of Li20-B203-WO3 system consists of both acidic area with Li20 content lower than 35 tool% and basic area with Li20 content higher than that. There are W 5+ and W 6+ ions coexisting in the glasses in the acidic area and there are predominately W 6+ ions in the glasses in the basic area. In the acidic area, the optical absorption of glasses from W 5+ ions decreases with increasing Li20 content and sharply increases with increasing WO3 content. The spin density of glasses decreases with Li20 content, which rapidly decreases with increasing WO3 content, in contrast with their optical absorption properties. It follows that when WO3 content increases polymerization of tungsten-oxygen polyhedra occurs and therefore, there must exist two electrons with paired spins in the 5d-state of tungsten ions in the larger tungstate clusters. The increase in W 5+ ions and the polymerization of tungsrate groups result in high electronic conductivity of the glasses.

Acknowledgement The authors gratefully acknowledge the support of the National Science Foundation of Chinese Academy of Sciences.

References

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hv(ev) Fig. 5. The plot of absorption coefficient of 20LizO.75B203.5WO3 glass versus photon energy.

[ 1 ] H.L. Tuller, D.P. Button and D.R. Uhlmann, J. Non-Cryst. Solids 40 (1980) 93. [2] F. Bonino, L.P. Bicell, B. Rivolta, M. Lazzari and F. Festorazzi, Solid State Ionics 17 ( 1985 ) 1. [ 3 ] P. Huang, X. Huang and F. Gan, Solid State Ionics 44 ( 1990 ) 11. [4 ] J.B. Wagner and C. Wagner, J. Chem. Phys. 26 ( 1957 ) 1597. [5] J.J. Kleperis, P.D. Cikmach and A.R. Lusis, Phys. Status Solidi (a) 83 (1984) 291. [6] F.P. Koffyberg, J. Non-Cryst. Solids 28 (1978) 241. [7] Y.R. Zakis, A.R. Lusis and Y.L. Lagzdonis, J. Non-Cryst. Solids 42 (1982) 267. [8] L. Kopp, B.N. H a r m o n and S.H. Liu, Solid State C o m m u n . 22 (1977) 677.